Protective fullerene (C60) packaging system for microelectromechanical systems applications

ABSTRACT

The invention involves a method for locating the probe of a scanning tunneling micrograph a predetermined distance from its conducting surface, and specifically the deposition of a monolayer of fullerene C 60  onto the conducting plate. The Fullerene C 60  molecule is approximately spherical, and a monolayer of fullerene has a thickness of one nanometer. By providing a monolayer of fullerene on the conducting surface and locating the probe on the surface of the monolayer, a distance of one nanometer can be established between the probe tip and the conducting surface.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. §202) in which the Contractor has elected not to retain title.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to microelectromechanical systems (MEMS), and more particularly to a protective packaging system and method whereby a one nanometer protective buffer is achieved using a monolayer of fullerene (C₆₀) to establish the preferred spacing of two components while protecting the components from contacting each other.

2. Description of Related Art

Fullerene C₆₀

Fullerenes are crystalline forms of carbon and are a relatively new discovery. A method to produce the C₆₀ fullerene, also known as “buckyballs, ” was first published by Kratschmer and Huffman in their article in Nature in 1990 (Vol. 347). The C₆₀ form of fullerene, depicted in FIG. 1, is comprised of sixty carbon atoms arranged to form a hollow, soccer ball-like sphere. Heretofore, fullerene has been used as a lubricant and scientists are striving to find novel uses for this unique substance.

Tunneling Tip Applications

Microelectromechanical systems, or MEMS, are miniature devices which are seeing their use in a wide variety of experimental and commercial applications. Tunneling tip MEMS have been demonstrated as being feasible for use as accelerometers, pressure sensors, seismometers, thermal sensors, and microphones among others. In the manufacture of MEMS, an electrically biased tunneling tip is used to drive electrons from the tunneling tip to a metal conducting plate. A MEMS tunneling tip device of this type can include a pyramidal metal tip that faces an electrically conducting plate or diaphragm across the tunneling gap.

The tunneling tip is positioned either manually or electrically to a preferred distance of one nanometer from the plate, and a bias voltage induces a current between the tunneling tip and the conducting plate as electrons are transferred from the tip to the plate. When a current is detected, the tip is assumed to be positioned one nanometer from the conducting plate. Currently, the technology which is used to position the tunneling tip relies on electrical feedback from the tip-plate system, but such feedback is difficult to maintain during fabrication and assembly. Both the tunneling tip and the conducting plate are typically gold because of gold's passive characteristics, but the use of gold poses a problem in positioning the tunneling tip with respect to the conducting plate. Accidental contact between the gold tunneling tip and the gold conducting plate can severely damage the components, in effect ruining the MEMS device. On occasion components which appear to have been successfully fabricated often were seen to have failed due to tip-plate crashing. Even where the tip is properly positioned relative to the conducting plate, the variation in sealing of the package is in many cases appreciable enough to cause the tip portion to be compromised thereby rendering the device inoperable.

SUMMARY OF THE INVENTION

The invention involves the deposition of a monolayer Of C₆₀ fullerene onto the conducting plate surface to protect the tip and conducting surface from premature contact (and subsequent damage). The Fullerene C₆₀ molecule is approximately spherical and bonds weakly to neighboring molecules. A monolayer of fullerene has a thickness of one nanometer, automatically establishing the theoretical distance desired between the MEMS' tunneling tip and the conducting plate. It would still be necessary to position the tip and diaphragm as in current fabrication process, but tunneling current would no longer be used to position the tip; instead, exploiting the electrical conductivity of C₆₀, one simply monitors for contact between the tip and the fullerene film as indicated by the onset of electrical conductance between them. By monitoring the conductivity between the tip and the fullerene layer as the tip is brought in proximity, the surfaces can be brought together without risk of contacting the conducting surfaces. Once the tunneling tip is positioned at the one nanometer spacing, with only the monolayer of fullerene between the tunneling tip and the conducting plate, the monolayer of C₆₀ can be broken down thermally and removed chemically leaving only the tunneling tip and the conducting plate at the ideal tunneling spacing. Alternately, the monolayer of fullerene can be left in place and the tunneling operation can occur directly across the fullerene cage.

BRIEF DESCRIPTION OF THE DRAWINGS

The exact nature of this invention, as well as its objects and advantages, will become readily apparent upon reference to the following detailed description when considered in conjunction with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof, and wherein:

FIG. 1 is a depiction of a Fullerene C₆₀ molecule;

FIG. 2 is a schematic of a tunneling tip contacting a monolayer of fullerene;

FIG. 3 is a schematic of a step in which energy is transmitted to the fullerene monolayer;

FIG. 4 is a schematic of breakup of the fullerene monolayer due to the energy transmitted,

FIG. 5 is a schematic of an introduction of a suitable gas in the region of the fullerene monolayer byproducts;

FIG. 6 is a schematic of the interaction of the gas with the byproducts; and

FIG. 7 is a schematic of the resulting spacing of the tunneling tip after evacuation of the gases.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide protective padding for microelectromechanical systems which advantageously establishes a protective layer having a critical spacing for tunneling applications.

Current MEMS technology requires tunneling tips and conductive surfaces such as diaphragms to be manually moved together until a tunneling current is established. At this point, the distance between the tip and diaphragm is theoretically one nanometer. Rather than manually setting the device by physically moving the two surfaces close together, the present invention teaches the use of a removable spacer layer to protect the tunneling tip from crashing into the diaphragm. Using fullerene C₆₀ as the spacer layer, an exact one nanometer distance can be established due to the molecular diameter of the C₆₀ cage being exacting one nanometer. Once the microelectromechanical system has been built, thermal and electrical energy can be used to cause molecular breakdown of the fullerene layer. Since studies have shown that under certain energetic situations fullerenes exhibit an “antifuse” property, it is possible to destroy the fullerene protective layer once the device has been deployed in its intended environments. Since the diaphragm surface would then be effectively contaminated with the residue of the non-destroyed protective layer, the surface may be treated to remove the carbonaceous soot and graphite left behind. Using thermal desorption and gettering of the newly created gasses to another surface, the contaminants are removed from the tunneling tip region. This can be performed either immediately after fabrication or after the component has been safely delivered to its destination.

As shown in FIG. 2, the first step is to deposit a monolayer 20 of C₆₀ fullerene on the conductive surface of a MEMS diaphragm. It has been demostrated that C₆₀ may be deposited onto a gold surface via sublimation. Additionally, C₆₀ is weakly chemisorbed on gold surfaces at room temperatures; however, bonding can be increased by annealing the surface if initial bond strengths are insufficient.

Once the protective layer has been deposited, the microelectromechanical device can be assembled as is normally done. Rather than monitoring tunneling current to determine optimized tip distance from the diaphragm, monitoring conductivity (indicated schematically by arrow 40) will also reveal the ideal distance since the diameter of C₆₀ is exactly one nanometer and the molecules are electrically conductive.

Once the tunneling tip 10 is positioned, the monolayer, as shown in FIG. 3, may be removed through the applications of thermal and/or electrical energy to the substrate, indicated by arrows 60. Where the energy of desorption of fullerene to the substrate surface is greater than the thermal input to the substrate, a bias created between the tunneling tip and substrate initiates molecular fragmentation 50 of the fullerene system as shown in FIGS. 4 and 5. Due to the relatively good electrical conductivity of fullerenes, a sufficient region around the tip would become electrically charged as the current is passed from the tip to the diaphragm surface. After a critical amount of thermal and/or electrical energy is supplied to the fullerene film, molecular breakdown of the fullerene will yield graphite and soot in the region of the tip.

With the substrate surface 30, and the carbonaceous contamination still thermally energetic, a dose amount of gas 70 is released (FIG. 5). The gas, which may be oxygen, hydrogen, or any suitable gas which reacts with the carbon byproducts of the prior step, is supplied by a pipe channel (not shown) sealed by a one event valve. These single event pipes and valves are well known and commonly fabricated in microelectromechanical systems.

After the device has reached its destination, the fullerene layer 20 can be energized with the thermal and/or electrical energy. The one event valve can then be opened, and the gas within the pipe fills the cavity between the diaphragm and the tip 10. The resulting interaction of the gas 70 with the carbonaceous contaminants 50 reacts to form a stable molecular gas 80, such as carbon monoxide and carbon dioxide, represented by CO and CO₂ in FIG. 6. Using a prefabricated sacrificial surface away from the tunneling device, these gases can be drawn to preferentially adsorb onto the sacrificial surface, leaving the region around the tunneling tip region free of adsorbed gases, fullerenes, or carbonaceous contaminants as shown in FIG. 7. The process thus not only provides one nanometer padding to protect the delicate components, but also ensures exact spacing necessary for the optimum operation of the system.

A key characteristic of C₆₀ is its robust and highly elastic nature which permit multiple cycles of operation, and it can reversibly tolerate a force in excess of 10 nano-Newtons. When connected between a metallic surface and a metallic tunneling tip, C₆₀ displays linear current-voltage characteristics. For bias much lower than the applied force of 1 nN, a 0.14 nm decrease in the diameter of the fullerene molecular cage (which is 7.1 Å without any force) is induced. Thus, if a single fullerene molecule is introduced in the junction, it can act as a mechanical buffer stabilizing the junction, permitting multiple operation cycles without tip and diaphragm modifications. Furthermore, the C₆₀ in the gap can also serve as the mechanism for electron transport from the tip to the conducting surface. In other words, the electrical conductivity of the fullerene supports electron transport such that, if desired, the fullerene monolayer does not have to be removed prior to the tunneling operation. There describes virtual resonance tunneling, and the variation of the current density flowing through the junction is two orders of magnitude per angstrom. A subnanometer variation in the tip altitude can be obtained by applying several millivolts to the piezoelectric element connected to the end of the tip. Consequently, a mechanically adjustable junction formed by the tip-C₆₀-metallic surface is an electrical circuit element that amplifies a signal applied to the piezo bias voltage. A fullerene molecule is especially suited for this gap because it has the proper spacing of approximately one nanometer, and further because it was found that moderate compression does not force the fullerene molecules to escape the tip by lateral motion nor change its overall configuration.

Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein. 

What is claimed is:
 1. A removable packaging method for establishing a one nanometer spacing between electrically conducting components comprising the steps of: depositing a monolayer of C₆₀ fullerene on a first fixed conductive surface; moving a second conductive surface with an electrical bias adjacent to the first conductive surface at a location where current is transferred to the monolayer of C₆₀ fullerene; breaking down the fullerene C₆₀ into carbonaceous byproducts; introducing a gas selected to react with the carbonaceous byproducts to form a stable molecular gas; and providing a sacrificial surface to selectively adsorb the stable molecular gas.
 2. A method for inhibiting contact between a tunneling tip and a conducting substrate comprising the steps of: depositing a monolayer of fullerene C₆₀ on the conducting substrate; providing the tunneling tip with an electrical bias; moving the tunneling tip to a position adjacent the conducting substrate and fixing the tunneling tip position with respect to the conducting substrate when an electrical current is detected in the fullerene C₆₀ monolayer due to the presence of the tunneling tip; energizing the monolayer of fullerene C₆₀ to breakup the monolayer into carbonaceous byproducts; and removing the carbonaceous byproducts, leaving the tunneling tip fixed at said fixed position.
 3. A method for manufacturing a MEMS device with a protective coating comprising the steps of: depositing via sublimation a fullerene layer onto a gold surface of a conducting substrate; providing an electrical bias on a gold-plated tunneling tip; moving the tunneling tip towards the conducting substrate, and monitoring conductivity in the fullerene layer; fixing the position of the tunneling tip when the monitoring of the conductivity in the fullerene layer indicates that the tunneling tip is in contact with the fullerene layer; applying thermal energy to the fullerene layer after the tunneling tip position has been fixed to break up the fullerene layer into carbonaceous byproducts; introducing a gas to react with the carbonaceous byproducts to form a carbon containing gas; and adsorbing the carbon containing gas onto a prefabricated sacrificial surface leaving a region adjacent the tunneling tip free of fullerene and fullerene byproducts. 